Rotala sp. “Vietnam”
The topmost leaves are translucent and some bend downward against the normal growth. The red stems lost their color. These symptoms were observed less than a day after dosing less than 1/120 tsp of Borax, Na₂B₄O₇*10H₂O, into my 60 gallon tank. Two days later, the growing tips died off. A couple of more days after that, the plants broke off mid-stem. These symptoms may be the result of a boron toxicity.
Also interesting is that these same symptoms resulted when I dosed potassium silicate, K₂O₃Si. Since silicon and boron are utlized for proper cell wall structure, excess or deficiency directly affects its growth.
The concentration at which an element becomes toxic is not absolute – it is often relative to all other elements. In this case, since I was dosing extremely lean traces, the excess boron probably resulted in toxicity due to it’s relatively high concentration. Thus, any statement that a certain concentration of an element is safe or toxic cannot be made without knowing what this concentration is relative to.
Relevant posts:
Update: Rotala “Vietnam” boron toxicity
Limnophila aromatica boron toxicity
Classical Aquascaping 